1. Field of the Invention
The field of this invention relates to a circuit, a communication unit, and a method for adjusting a voltage controlled oscillator (VCO) frequency, and in particular to a circuit, a communication unit, and a method for pushing or load pulling a VCO frequency to avoid 4FMod spurious signals.
2. Description of the Prior Art
In telecommunications, there has been a recent trend for device manufacturers to design wireless communication units that are capable of operating over multiple frequency bands, to enable the same device to operate in different geographical regions, as well as being able to switch between different service providers and different communication technologies. All wireless/mobile phone communication devices that support communications on more than one channel use multiple frequencies.
Hence, in the field of Radio Frequency (RF) communication units, architectures for supporting communications across multiple and various frequencies have been developed. Typically, a single architecture is able to support multiple frequencies through provision of variable, programmable, frequency generation signals, for example for Long Term Evolution (LTE™) communication bands 13 and 26.
In LTE™, there are many types of signal configurations and some of the signal configurations have wide bandwidth and are centered on the carrier frequency; whereas other signal configurations are not. For example, in LTE™ uplink, signals with certain assigned resource block configurations can occupy frequency spectrum that is offset from the carrier frequency (in a similar manner to the known technique of Single Side Band (SSB) modulation). These signals can be similar to Continuous Wave (CW) tones that can be offset from the carrier frequency by as little as 180 kHz, which can be a significant offset for Phase Locked Loop (PLL) frequency pulling.
FIG. 1 illustrates a simplified block diagram of a basic PLL 100 comprising phase detector 110, loop filter 120, voltage controlled oscillator 130 and divider 140. The phase detector 110 compares input signal 105 with reference signal 106 output from the divider 140, to produce an error signal 107 that is proportional to the phase difference between signals 105 and 106. The loop filter 120 extracts the low frequency content of the error signal 107, which is input into the voltage controlled oscillator 130. The voltage controlled oscillator produces a change in its output signal 108 that is proportional to the error signal 107. The change in output signal 108 is also typically divided by a divider producing the reference signal 106. By feeding back this reference signal 106 into the phase detector 110, a closed loop system is formed that ensures the input signal 105 has the same phase as the reference signal 106.
In some instances, these signals can also be spaced almost 10 MHz away from the carrier. In such a case, harmonics of the modulated signals can inter-modulate with the desired signal in a Power Amplifier (PA) driver stage, thereby creating out-of-band spurious products. As illustrated in the simplified frequency response representation of FIG. 2, such out-of-band spurious products may include a primary 4FMOD spurious signal 210 that is due to an up-converted third harmonic of a baseband signal mixing with the Local Oscillator (LO) signal. A secondary 4FMOD spur may also be created, which is an image of the primary and is therefore much weaker. In frequency, the primary 4FMOD signal is always on the other side of the LO signal from the desired signal. If the frequency difference between the LO signal and the desired signal is denoted ‘F’, then the frequency difference between the desired signal 220 and the primary 4FMOD signal is 4F. The current known technique to avoid creating a problematic 4FMOD signal, as explained in US 2011/0143697 A1, is to modify the up-mixer to function as an harmonic rejection mixer.
Orthogonal Frequency Division Modulation (OFDM) in LTE20 creates up to one hundred Resource Blocks (RBs), each containing twelve subcarriers with 15 kHz spacing, spread over the channel bandwidth. For an RB with frequency offset fs from the Local Oscillator (LO), transmitter non-linearity causes counter-intermodulation (CIM) products at odd multiples of the frequency offset fs: −3fs, +5fs, −7fs, . . . from the LO. These spurious signals, either directly or due to re-mixing due to Power Amplifier (PA) non-linearity, may fall on other frequency bands. For example, when a strong signal is present at the PA output 220 at an offset equal to (half) the bandwidth (BW) of PLL 230, very strong pulling 240 of the frequency occurs, and the PLL is much less effective in resisting it. A known solution to this problem is to increase the PLL BW. However, such a solution increases the output noise. The worst case is when one or just a few RBs are transmitted at the edge of the channel. CIM terms are first generated by mixers. For example, a Drive Amplifier (DA) placed after the mixers will cause these terms to be regenerated due to high-level harmonic sidebands. Counter Intermodulation problem is typically solved by use of harmonic reject mixer which will reduce the third harmonic at the input of the PA.
An oscillator is sensitive to its supply voltage. A change in supply voltage changes the output frequency, and is typically referred to as VCO frequency pushing. The output power at the PA causes significant current to flow between the supply voltage and ground. Some of this current flow changes the VCO supply, and this happens at the rate of modulation, which can be many MHz. In such a scenario, the PLL is unable to correct for it. A pushing factor is defined as the output frequency change in Hz per volts of supply voltage change (normally expressed as MHz/V).
In contrast, a pulling factor of a VCO quantifies its susceptibility to output load variations. This is a slow process and typically the PLL can act fast enough to correct for this impairment. If the output load changes, the oscillator frequency changes in response. A variation of the output load of the VCO changes its Voltage Standing Wave Ratio (VSWR) performance, as well as the return loss. In some examples, the pulling factor may be minimized by using a buffered output. Typically, analog circuits adjust the oscillator's supply voltage are used to combat PLL pulling. VCO pushing degrades the output spectrum and is typically solved by increasing the PLL BW or reducing the magnitude of perturbation of VCO supply due to output.
There is a general need for concepts to avoid Radio Frequency (RF) impairments, particularly with relation to LTE carrier frequencies and problems associated with VCO pushing and/or 4Fmod.